The chief objectives of the program are to analyze genetic and epigenetic mechanisms in mammalian differentiation, genetic diseases, and malignancy. Several experimental systems are being utilized. One of these depends upon the capacity of mouse teratocarcinoma stem cells to lose their malignancy and participate in development when microinjected into early-stage mouse embryos. In the best cases, germinal derivatives are formed; thus, progeny with the tumor-lineage genes are obtained. The tumor stem cells may therefore serve as vehicles for introducing into mice predetermined mutations, including some designed to yield laboratory models of human genetic diseases: Teratocarcinoma cells, after mutagenesis during in vitro culture, are selected or screened to obtain clones with the desired mutation, and the mutant cells are then injected into embryos. In another approach, recombinant cloned genes are being introduced either into the teratocarcinoma cells or directly into embryos, in order to analyze the relationship between gene organization and tissue-specific control of gene expression. An additional experimental system has recently been devised here for investigating genetic and developmental mechanisms involved in hematopoiesis: This depends upon early prenatal substitution of the hematopoietic cell lineage with genetically marked hematopoietic stem cells.